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I don't work in the disc industry however...
There are differences in the types of plastics that are used. Disc manufacturers are at the lower end of the plastic industry (by use) so the plastic they can buy today may not be the plastic that they can buy a month from now. There can be slight changes in the mix.
The discs are not molded in an atmospheric controlled climate. Daily/Seasonal changes in temperature, humidity, etc affect how the disc comes out of the mold.
There there is the slight changes that are made to molds from time to time such as the Beast. The Beast the you buy today is not the same mold as the one from a few years back.
Warning!! I am a plastics student, not a disc manufacturer, and have never worked for one but that would be great! Views and opinions are not necessarily those of the disc manufactures read at your own risk. These are my perceptions based on what I have learned in plastic engineering courses. Every plastic manufacturing facility does things a little different.
First off what is thermoplastic?
A thermoplastic is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers. Thermoplastic polymers differ from thermosetting polymers (Bakelite; vulcanized rubber) as they can, unlike thermosetting polymers, be remelted and remoulded such as in recycling.
Ok, so our discs are made from thermoplastic, big deal, why are the runs so different? In any production injection molding facility they need to have lots of raw material for their product. Most get it delivered by rail or truck then have to store it somewhere. Most plastic manufactures store the material in outdoor silos and convey the material into the building. This all can create your first problem. Some thermoplastic material is hygroscopic (hy.gro.scop.ik), readily taking up and retaining moisture, or moisture taken up and retained under some conditions of humidity and temperature. So basically if it was delivered when it was cold or humid or stored outside in a silo where it is cool, then conveyed into a warm building or machine the material will begin to sweat, moisture in an injection molding process is bad for a few reasons. Most companies have air diffusers (dryers) in the machine hoppers to attempt to rid the pellets of any moisture but it is still possible to have a moisture problem.
First Run, the mold that we will call mold “1” is set in place onto, we’ll call it machine “A”. From here the process is setup, machine adjustments made for this particular mold as this machine was running mold “5” yesterday or last shift. Many adjustments on injection molding equipment can depend on the size of the machine, specific plastic being used, size of the mold cavity, ambient air temperature…etc. They will run test shots to determine when the part is a quality part, once they have achieved this they will begin to run part after part while monitoring part quality and machine settings. First runs I think usually consist of around 1000-2000 discs but that could be wrong. Either way after they are done for the shift and are going to be running mold “3” on this machine next shift so they will be changing mold “1” out for “3” and beginning the set up process all over again.
So why can’t they just use the same machine settings? Let’s say a week later they are going to be running mold “1” on machine “B”. Also, let's say the mold was slightly damaged (scratched) inside of the mold cavity somehow and had to be slightly ground and sanded to restore the glassy smooth surface inside the cavity area.The weather has gotten very hot the production area is 98*F, they are on a new batch or “lot” of material, even though it is the same material it was produced at a separate time. They can get close with the same settings, but in injection molding each setup needs to be fine tuned for the current atmosphere and machine being used. How many people do you have setting up processes? If you have first, second, third shifts, you probably have at least one for each shift. This can bring human error into the equation. This can be true not only with machine setup but also with the processing itself. Not to say they aren’t making quality parts, but their perception of a quality part can differ, many factors can give the part different characteristics, some that you will not see on the part itself but in the flight characteristics.